1. Field of Invention
The present invention relates to porous glass, particularly optical fibers, and more particularly to a porous glass optical fiber sensor made for use in a detection system for sensing gas or liquid agents The porous structure of the glass, which is made by first forming a phase-separable glass fiber and then exposing the fiber to selective heat treatment, phase separation and leaching, has a very high surface area. The agent(s) of interest for sensing can permeate the porous structure and be optically detected. A chemical indicator can be used to coat the porous structure for certain optical detections. The optical fiber is used in an electronic detection system which compares light input to the fiber with light output from the fiber to sense the agent(s) of interest within the porous structure of the fiber.
2. Description of the Related Art
U.S. Pat. No. 2,315,328 ("HOOD") is directed toward a high silica glass article. HOOD discusses a phase separation and leaching technique to form a permeable, porous borosilicate glass. HOOD also describes impregnating the glass with dyes, pigments, resins, and various other chemicals depending upon the intended function. The patent further describes certain catalytic properties of porous glass in "any suitable shape . . . such as . . . small hollow cylinders which may be impregnated with various catalysts and used for the promotion of chemical reactions." Column 5, lines 35-50. The catalytic reactions are both reversible and sensitive. HOOD further makes reference to impregnating or filling the pores with a resin or viscous, high boiling temperature liquid which has a substantially different refractive index than that of the glass. Column 3, lines 37-40.
U.S. Pat. No. 3,272,646 ("CHOPOORIAN") concerns an impregnated porous photochromic glass. CHOPOORIAN describes a colorless, transparent, variable transmission porous glass, having the pores impregnated with a solution of an aromatic diaminetetraacetic acid, which darkens in the presence of ultraviolet light. Column 1, lines 14-20. The CHOPOORIAN patent also teaches entrapping the acid in the pores by coating the porous member with a film. Column 4, lines 30-63.
U.S. Pat. No. 3,904,422 ("EATON") describes the use of heat treating, phase separation and leaching to form porous glass which is used, because of its inherent properties of extreme inertness, optical transparency, and large surface area, as a chromatographic separation medium.
U.S. Pat. No. 3,938,974 ("MACEDO '974") concerns a method of producing optical waveguide fibers. The MACEDO '974 patent shows a phase-separable glass converted to a porous form. The form is substantially made of silica and can then be converted to a solid glass article. One aspect of the invention is the incorporation of a chemical dopant into the interconnected pores. The dopant is used to modify the index of refraction of the glass. Column 3 line 50 to column 4 line 2.
U.S. Pat. No. 4,097,258 ("HORIKAWA") concerns an optical fiber. HORIKAWA teaches heat treatment, phase separation and leaching to produce a porous object which is sintered to produce a core for an optical fiber. We believe that the disclosed process of sintering and drawing the optical fiber removes the porosity.
U.S. Pat. No. 4,220,461 ("SAMANTA") is titled "Low Temperature Synthesis of Vitreous Bodies and Their Intermediates." SAMANTA provides a historical perspective and discussion on the development of silica-rich phase-separable porous glass. SAMANTA is concerned with depositing layers of glass having different porosities, particularly to form a membrane that is permeable to one substance, but impermeable to another substance.
U.S. Pat. No. 4,236,930 ("MACEDO '950") discusses an optical waveguide and method and compositions for producing the same. Optical fibers are produced by locating a dopant within the porosity of a glass form, collapsing the form, and producing an optical fiber. The main emphasis is on chemical composition differences needed to influence the index of refraction.
U.S. Pat. No. 4,657,875 ("NAKASHIMA") describes articles of porous glass and process for preparing the same. NAKASHIMA teaches methods for producing porous glass by phase separation and inorganic acid leaching. The improvements involve chemical composition changes. NAKASHIMA discusses a tube-like structure in examples 1-3.
U.S. Pat. No. 4,665,039 ("KOKUBU") is directed towards porous glass, a process for its production and glass material used for the production. KOKUBU describes heat treatment to induce phase separation and subsequent leaching to form porous glass. A porous form about 1 mm thick is discussed at column 5, line 10, etc.
Fiber optic sensors have also been previously considered in the art. For example, fiber optic sensors have been used to determine the humidity of air. The use of an optical fiber evanescent sensor for humidity measurements is described by A. P. Russell and K. S. Fletcher in Anal. Chim. Acata 170, 209 (1985). In their device, a moisture sensitive cobalt chloride/gelatin film is immobilized on a 12 cm long silica optical fiber for use as a humidity probe. David S. Ballantine and Hank Wohltjen describe an optical waveguide humidity sensor that employed the same colormetric reagent/polymer system on glass capillary. Anal. Chem. 58, 2883 (1986). A fluorescent fiber optical sensor for atmospheric humidity, based on a dye entrapped with a polymer matrix, is discussed by Chu Zhu and Gary M. Hieftje, Abstract 606, paper presented at the Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectrocopy, Atlantic City, N.J., 1987. However, all these sensors have considerable sensitivity limitations.
Waveguide fibers have also been coated with substances to detect ammonia vapor concentrations. D. J. David, M. C. Wilson and D. S. Ruffin, "Direct Measurement of Ammonia in Ambient Air", Anal. Lett. 9, 38 (1976), and T. A. Orofino, D. J. Dand and E. E. Hardy, "A Technique for Work-Station Monitoring Utilizing Optical Waveguides", presented at the fourth joint conference on Sensing Environmental Pollutants, New Orleans, La. 1977 have both described a probe consisting of a quartz glass lightguide coated with a pH indicator (ninhydrin) that could detect ammonia vapor concentrations of below 100 ppb. However, the dye reaction is irreversible and of limited practical use. J. F. Giuliani, H. Wohltjen and N. L. Jarvis, "Reversible Optical Waveguide Sensor for Ammonia Vapors", Optic's Letters, Vol. 8, No. 1, Jan. 1983 reports a reversible sensor for ammonia gas consisting of a 90 mm long commercial soda-glass capillary tube coated with an oxazine perchlorate dye. The lowest detectable ammonia concentration reported is about 10 ppm.
Nonetheless, the prior art has not disclosed a reliable, reversible, versatile, inexpensive and miniaturizable device for chemical sensing. Lack of sensitivity in detection has also been a consistent problem in the prior art.